Even on a night which is too dark for natural human vision, invisible infrared light is richly provided by the stars. Human vision cannot utilize this infrared night-time light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. Under such conditions, a night vision device of the light amplification type can provide a visible image replicating the night time scene. Such night vision devices generally include an objective lens which focuses invisible infrared light from the night-time scene onto the transparent light-receiving face of an image intensifier tube (I.sup.2 T). At its opposite image-face, the I.sup.2 T provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
A contemporary night vision device will generally use an I.sup.2 T with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of dynodes, or microchannels. These dynodes or microchannels have an interior surface substantially defined by a material of high emmisivity of secondary electrons. In other words, each time an electron (whether a photoelectron or an electron previously emitted by the microchannel plate) collides with this interior surface material, more than one electron (i.e., secondary-emission electrons) leaves the site of the collision. This process of secondary electron emissions is not an absolute in each case, but is a statistical process. The photoelectrons entering the microchannels thus cause a geometric cascade of secondary-emission electrons moving along the microchannels so that a spatial output pattern of electrons which replicates an input pattern (but at a considerably higher electron density than the input pattern) issues from the microchannel plate. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen electrode by another electrostatic field. When the electron shower from the photocathode impacts on and is absorbed by the phosphorescent screen electrode, a visible image is produced. This visible image is passed out of the tube through a transparent window for viewing.
A conventional power supply for a conventional I.sup.2 T provides the electrostatic field potentials referred to above, and also provides a field and current flow to the microchannel plate(s). This power supply provides the necessary voltage levels to provide the required electrostatic fields maintained within the image intensifier tube to move electrons from the photocathode to the screen electrode. Unavoidably, these electrostatic fields also move any positive ions which exist within the image intensifier tube toward the photocathode. Because such positive ions may include gas atoms of considerable size, they are able to impact upon and cause damage to the photocathode of many conventional image intensifier tubes. This impact of positive ions on the photocathode contributes to a relatively short operating life for many early-generation image intensifier tubes. As those ordinarily skilled in the pertinent arts will understand, later generation image intensifier tubes of the proximity focus type have partially solved this ion-impact problem by providing an ion barrier film on the inlet side of the microchannel plate.
This ion barrier film itself, however, is not without disadvantages. A recognized disadvantage of such an ion barrier film on a microchannel plate is the decreased gain provided by the microchannel plate between a photocathode of an image intensifier tube and the output screen electrode of the tube. That is, the ion barrier film also acts as a barrier preventing some of the photoelectrons liberated from the photocathode of the tube from reaching the microchannels of the microchannel plate. In some cases, as much as 50% of the electrons liberated from the photocathode of the I.sup.2 T and approaching the microchannel plate will be blocked and will not reach the microchannels to be amplified as described above. Thus, about the same percentage of the image information which theoretically could be provided by this tube is lost.
U.S. Pat. No. 3,720,535, issued Mar. 13, 1973; U.S. Pat. No. 3,742,224, issued Jun. 26, 1973; and U.S. Pat. No. 3,777,201, issued Dec. 4, 1973 provide examples of microchannel plates or image intensifier tubes having an ion barrier film on a microchannel plate.
Further to the above, conventional night vision devices (i.e., since the 1970's and to the present day) provide several protective functions. One of these protective functions is referred to as "automatic brightness control" (ABC), and another protective function is called "bright source protection" (BSP). The ABC function maintains the brightness of the image provided to the user substantially constant despite changes in the brightness (in the infrared and near-infrared portion of the spectrum) of the scene being viewed. BSP prevents the I.sup.2 T from being damaged by an excessively high current level in the event that a bright source, such as a flare or fire, comes into the field of view. These functions are somewhat analogous to area and spot exposure controls on a camera. ABC controls the image brightness using the entire area of the viewed scene, while BSP uses light emission levels from a spot in the scene (which need not be centered in the scene) in order to provide the protective function.
The ABC function is conventionally accomplished by providing a regulator circuit monitoring the output current from the phosphorescent screen electrode (See FIG. 9). When this current exceeds a certain threshold, the field voltage level across the opposite faces of the microchannel plate(s) is decreased to reduce the gain of the microchannel plate(s), as is graphically depicted in FIG. 10. This reduction of microchannel plate voltage also has the effect of reducing the resolution of the I.sup.2 T. That is, the gain versus voltage function of the I.sup.2 T at lowered MCP voltages results in a matrix pattern from the microchannel plate(s) appearing in the image. This matrix pattern is sometimes referred to as fixed-pattern noise in the image.
As a result, in bright-field conditions with the ABC feature of a conventional night vision device operating the conventional night vision device may drastically lose resolution so that the user of the device is no longer able to discern details of the viewed scene which would be discernible were they viewed under darker field conditions in which ABC were not applying.
BSP is provided in conventional night vision devices by decreasing the field voltage provided to the photocathode. This voltage reduction happens in the conventional power supply circuit because when a bright object appears in the viewed scene a large number of photons will be incident on an area of the photocathode, and the high impedance of the photocathode in combination with a high resistance value circuit element creates a greater voltage drop under these high current conditions. The large number of photoelectrons provided by the photocathode under these conditions represent a current flow increasing in magnitude with increasing light levels in the viewed field, such that the combined impedance of the photocathode and circuit element causes a decrease in the voltage level effective at the photocathode to move these electrons to the microchannel plate(s).
Recalling FIG. 9, it will be noted that the circuit architecture of the prior art uses two transformers, which are relatively large and heavy components of the circuit. Further, is seen that a typical conventional circuit architecture for a power supply of a night vision device provides a high-value resistor (generally 1-18 G-ohm) to the output of the photocathode voltage multiplier, and a clamping circuit consisting of a voltage source and a low-leakage, high-voltage diode. As photocathode current flows through the high-value resistor, the photocathode voltage will decrease linearly until it reaches a voltage equal to the voltage source (plus the high-voltage low-leakage diode voltage drop). See FIG. 11 for a graphical illustration of this BSP voltage relationship at the photocathode. This voltage is commonly referred to as a clamp voltage, and is typically between 30 and 40 volts D.C.
This conventional method of BSP also has a disadvantage of decreased resolution for the I.sup.2 T. The reduced electrostatic field between the photocathode and the microchannel plate(s) input causes a reduced resolution for the tube. That is, photoelectrons liberated from the photocathode are not moved to the microchannel plate(s) as effectively, and may not be liberated to reach the microchannel plate(s) at all. This is because photoelectrons must overcome a surface potential barrier at the photocathode in order to be liberated into free space (i.e., into a vacuum within the image intensifier tube), and to be moved by the prevailing electrostatic field to the input of the microchannel plate(s). As the voltage applied to the photocathode decreases (again viewed statistically), some photoelectrons will not be able to overcome this surface potential barrier and will not be liberated into free space. The image information represented by these trapped photoelectrons will be lost from the image provided by the I.sup.2 T to the user of the night vision device.